This application is based on and incorporates herein by reference Japanese Patent Applications NO. 2001-27810, NO. 2001-27811 and NO. 2001-27812, all filed on Feb. 5, 2001.
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine for feedback-controlling the air-fuel ratio of the internal combustion engine by air-fuel ratio sensors (linear A/F sensor) or oxygen sensors which are disposed on the upstream side and the downstream-side of a catalyst for cleaning exhaust gas, respectively.
In automobiles, exhaust gas sensors (air-fuel ratio sensors or oxygen sensors) are disposed at the upstream and downstream sides of a catalyst to feedback-control a fuel injection amount so that the air-fuel ratio detected by the upstream-side exhaust gas sensor becomes an upstream-side target air-fuel ratio. A sub-feedback control is performed, by which the upstream-side target air-fuel ratio is corrected so that the air-fuel ratio detected by the downstream-side exhaust gas sensor becomes equal to a downstream-side target air-fuel ratio.
In such a main/sub-feedback system, it is proposed in Japanese Patent NO. 2518247 that as the deviation between the air-fuel ratio detected by the downstream-side exhaust gas sensor and the downstream-side target air-fuel ratio becomes larger, the update amount of an air-fuel ratio feedback control constant is increased.
The dynamic characteristics of the catalyst varies depending on the degree of deterioration of the catalyst, the state of adsorption of the lean/rich components in the catalyst, and the state of operation of an engine. In this main/sub-feedback control system, the response of the sub-feedback control to a change in the dynamic characteristics of the catalyst is not sufficient. Thus, a delay in the response of the sub-feedback control to the change in dynamic characteristics of the catalyst occurs. Thus, the air-fuel ratio on the downstream-side of the catalyst (output of the downstream-side exhaust gas sensor) becomes unstable and tends to fluctuate.
Therefore, it is proposed in U.S. application Ser. No. 09/838591 filed on Apr. 20, 2001 (Japanese Patent Application NO. 2000-464671) to set an intermediate target value of the sub-feedback control on the basis of the air-fuel ratio previously detected by the downstream-side exhaust gas sensor and a final downstream-side target air-fuel ratio and to perform the sub-feedback control for correcting an upstream-side air-fuel ratio on the basis of the deviation between the air-fuel ratio detected by the downstream-side exhaust gas sensor and the intermediate target value.
In this system, a three-way catalyst used for cleaning an exhaust gas cleans the exhaust gas by oxidizing or reducing rich components (HC, CO, etc.) and lean components (NOx, oxygen, etc.) in the exhaust gas or by making the catalyst adsorb the rich components and lean components in the exhaust gas. When the exhaust gas continues to be biased to a lean or rich state, the amount of the lean components or the rich components adsorbed by the catalyst increases and finally the adsorption amount of the catalyst becomes saturated. When the catalyst becomes the state of saturated adsorption, the air-fuel ratio on the upstream-side of the catalyst is controlled by a sub-feedback control in the direction which reduces the adsorption amount of the catalyst. During a period from the state where the catalyst is in the state of saturated adsorption to the state where the catalyst is returned to the state of insufficient adsorption, however, the storage state of the catalyst is unstable. If the sub-feedback control with high response using an intermediate target value is performed under the same conditions as in a normal state, the sub-feedback control is becomes unstable and causes over-shooting or fluctuation which results in increasing uncleaned exhaust gas.
Further, the catalyst has a delay system (dead time and time constant) which largely varies depending on an exhaust gas flow and a catalyst reaction rate. In this case, if the intermediate target value used for the sub-feedback control is updated under slow conditions so as to prevent fluctuation, the intermediate target value is suitably updated in the case of a small exhaust gas flow or in the case of a slow catalyst reaction rate (in the case where the cleaning performance of the catalyst is reduced). However, in the case of a large exhaust gas flow or in the case of a fast catalyst reaction rate, the update of the intermediate target value (response of the sub-feedback control) is too late to ensure a sufficient performance in cleaning the exhaust gas.
Still further, as the downstream-side exhaust gas sensor, an oxygen sensor (O2sensor) is used. This sensor output characteristic is inverted depending on whether the air-fuel ratio of the exhaust gas is rich or lean. The output characteristic of the oxygen sensor is referred to as a Z-characteristic. Specifically, in a region where an air-fuel ratio is near the stoichiometric air-fuel ratio region (excess air ratio xcex=1), that is, in a region where the output voltage of the oxygen sensor is from 0.3 V to 0.7 V, even if a change in an air-fuel ratio is small, the output voltage of the oxygen sensor changes largely. On the other hand, where the output voltage is in a rich region of 0.7 V or more or in a lean region of 0.3 V or less, a change in the output voltage of the oxygen sensor with respect to a change in the air-fuel ratio becomes small.
If the sub-feedback control is performed by setting the intermediate target value (intermediate target voltage) by using the output voltage of the oxygen sensor having the Z-type characteristic like this as it is, because a change in the output voltage of the oxygen sensor with respect to a change of the air-fuel ratio is small in a rich region of 0.7 V or more and a lean region of 0.3 V or less, the update amount of the intermediate target value (intermediate target voltage) is made small with respect to a change in an actual air-fuel ratio to delay the response of the sub-feedback control with respect to a change in the air-fuel ratio. Thus, the delay in the response increases the exhaust amount of HC, CO in the rich region of 0.7 V or more and the exhaust amount of NOx in the lean region of 0.3 V or less.
Still further, because a change in the output voltage of the oxygen sensor with respect to a change of the air-fuel ratio is steep in the region of the stoichiometric air-fuel ratio (from 0.3 to 0.7 V), the update amount of the intermediate target value (intermediate target voltage) is made too large with respect to a change in the air-fuel ratio. Thereby a fluctuation tends to occur in the sub-feedback control and reduce the stability of the sub-feedback control.
It is therefore an object of the present invention to provide an air-fuel ratio control apparatus which provides improved performance in exhaust gas cleaning.
According to the present invention, exhaust gas sensors are provided at the upstream side and the downstream side of a catalyst, respectively. An intermediate target value is set on the basis of the output of the downstream-side exhaust gas sensor of preceding computation time and a final target value that is a final downstream-side target air-fuel ratio. The compensation amount of the upstream-side target air-fuel ratio is calculated on the basis of the deviation between the present output of the downstream-side exhaust gas sensor and the intermediate target value. At least one of an update amount and an update rate of the intermediate value, a control gain, a control period and a control range of a sub-feedback control is varied.